Here is my explanation of TA(Timing Advance) concept in GSM.
The MS and BTS communicates with each other using the radio(electromagnetic)waves.
Timing Advance is a concept employed in GSM when the MS enters into DEDICATED MODE.
In IDLE MODE the Timing Advance has no significance.
What ever information MS transmits over a given TS reaches the BTS with a speed equal to the velocity of light(c). But even with this speed there occurs a delay. To address this delay the concept of Timing Advance has been evolved in GSM.
The definition of Timing Advance:
In the GSM cellular mobile phone standard, timing advance value corresponds to the length of time a signal from the mobile phone takes to reach the base station. GSM uses TDMA technology in the radio interface to share a single frequency between several users, assigning sequential time slots to the individual users sharing a frequency. Each user transmits periodically for less than one-eighth of the time within one of the eight time slots. Since the users are various distances from the base station and radio waves travel at the finite speed of light, the precise time at which the phone is allowed to transmit a burst of traffic within a time slot must be adjusted accordingly. Timing Advance (TA) is the variable controlling this adjustment.
Technical Specifications 3GPP TS 05.10 and TS 45.010 describe the TA value adjustment procedures. The TA value is normally between 0 and 63, with each step representing an advance of one symbol period (approximately 3.69 microseconds). With radio waves traveling at about 300,000,000 meters per second (that is 300 meters per microsecond), one TA step then represents a change in round-trip distance (twice the propagation range) of about 1,100 meters. This means that the TA value changes for each 550-meter change in the range between a mobile and the base station.